1. Field of the Invention
This invention relates generally to a heat retention device for containers and more specifically to heat storage and retention devices capable of absorbing thermal or microwave energy, storing it as latent heat energy in a material disposed in a chamber of the device that is isolated from the food stuffs, thereby maintaining the temperature of the food stuffs at an elevated temperature.
2. Background Art
Keeping food warm after its preparation and prior to its consumption has long been a desirable goal of food preparers. Especially in more recent times, following the recognition that to be safe, food must be free of bacteria and other unhealthy contaminants, food is required to be kept in a temperature range of less than about 38° F. (for which occasion assignees of the present invention have developed a corresponding construction, see commonly owned U.S. Pat. No. 4,989,419) and above about 140° F., the subject of this invention. Special attention to this problem is required for foods served in restaurants, to patients in hospitals, and other instances when a relatively long period of time elapses between the food preparation and the time the food is served and consumed. Additionally, it is also desirable to maintain the temperature of food that requires delivery over long distances, for example, pizza or take out food.
Another instance in which food should be kept warm is when it has been prepared for self service, for example, on an appetizer tray. Here, as the food is consumed over a period of time by persons serving themselves, there is normally a lapse of time between the food being ready and its actual consumption.
Microwave ovens have become standard appliances in most kitchens and food preparation areas. They use electromagnetic radiation to heat, in most instances, water molecules contained within food stuffs, and so to cook foods or warm them up for serving. A microwave oven utilizes very short radio waves, the so called microwaves, which are also commonly employed in other standard uses, such as radar and satellite communications. When concentrated and focused into a small volume, microwaves can efficiently heat water and other substances contained in that volume, such as foods. Microwaves generally cook food rapidly and efficiently because, unlike conventional ovens, they only heat, for example, water contained in the food, and no need exists to heat the air or the oven walls. Heat energy then disperses within the food by conduction from the heated water molecules.
Microwaves can easily pass through many types of materials, including heat insulating materials, for example, glass, paper, ceramics, and plastics. Containers made of these materials are thus usable for containing food. Various types of dishes are currently available in the marketplace and are adequate for the uses to which these items are required. One drawback to these types of dishes is that the materials from which they are made do not normally retain heat and nothing but the internal latent heat of the food exists to maintain the proper food temperature. After the initial heating in a microwave oven, a relatively large thermal gradient exists between the heated food and the environment, including the container material. Therefore, upon removal of the heated food from the microwave, the heat quickly dissipates from the food and transfers to the ambient environment and to the container, thereby reducing food temperature to below acceptable levels.
Past attempts to counteract the tendency of the food in a container to quickly cool include the use of materials that are able to retain some heat energy after the container and food is removed from the microwave oven. These materials are capable of absorbing and retaining the microwave radiation energy and then reradiating or conducting the energy as heat from the heat retention material to the food or to the walls of the food container that are in contact with the food. Such materials have included, for example, quarried soapstone (McCarton et al.; U.S. Pat. No. 4,258,695), wet sand (Sepahpur; U.S. Pat. No. 4,567,877), silicone rubber with entrained ferrite particles (U.S. Pat. No. 5,107,087), earthenware with entrained small iron filings or particles (Ramirez; U.S. Pat. No. 7,176,426), etc. While these and other materials are adequate for retaining heat energy that can be transferred to the food, the materials may not be palatable, and may indeed be unsafe for human consumption. Thus, many of the known food containers enclose the heat absorbing material in a sealed portion of the food warming container, mainly to isolate the food from the heat retaining material as a safety feature, and also to maintain the heat retaining material in place for future reuse.
Johnson, U.S. Pat. No. 5,052,369 teaches a heat retaining food container having a cover and a bottom portion, each one of the cover and bottom portion including a heat storage system comprised of a non-metallic heat storing mass enclosed within a sealed chamber. The walls forming the chamber are formed from a polymeric material, such as hard plastic, which is transparent to microwave radiation, is also physically and chemically stable up to approximately 400° F., is chemically stable to detergents and other rinsing agents, and is resistant to staining and discoloration.
One disadvantage of the heat retaining containers that are hermetically sealed, for example, such as that taught by Johnson, is that neither the cover nor the bottom portion include a safety valve for escape of gases that may be generated by overpressure during excessive heating of the material in the container. Such temperatures may be far in excess of those to which the container and material would be exposed in normal usage, and may even exceed those that might occur through accidental overheating.
When the microwave absorbing material contents is overheated, that is, it is heated beyond the time or power level necessary to achieve optimum latent heat retention, the pressure in a sealed chamber may become excessive and must be accommodated by the structure of the chamber in which the heat retaining material is disposed. Accidental, and sometimes malicious, overheating in a microwave oven, a frequent event, thus normally causes the container to crack, rupture or become permanently deformed. On occasion, the deformation is catastrophic because the high pressures developed in the chamber are contained until such a high pressure is reached that renders the container wall material susceptible to rupture or explosive stress fracture, thereby relieving the overpressure, sometimes in a violently destructive manner.
To overcome the risk of loss of structural integrity, some devices have a very robust construction so as to be capable of withstanding high temperature and pressure levels. For example, Murdough et al., U.S. Pat. No. 3,734,077, and Lanigan et al., U.S. Pat. No. 3,837,330, both teach that the danger of bursting is avoided by reason of the configuration and construction of the device, which utilizes a secure interconnection between the upper and lower portions of the shell containing the heated material.
Many devices provide means to overcome the destructive capacity of overpressurization of the containers due to overheating by including some pressure relief mechanism. For example, Ramirez U.S. Pat. No. 7,176,426, relies on using a solid, rather than fluid, heat retention material and also on minimizing the volume of air within the chamber by sealing the chamber at high temperatures so as to cause a semi vacuum, i.e., negative pressure. Because the volume of fluid material susceptible to expansion upon heating is minimized, gas within the chamber does not cause excessive pressures when overheated to a reasonable level. Others, for example, Wyatt, U.S. Pat. No. 6,005,233 and U.S. Pat. No. 6,188,053, teach an elaborate and complicated pressure relief system using one or more types of check valves that vent excess pressure built up within the heat storage chamber to the environment. These types of complicated and expensive devices, such as dish carriers (with their corresponding covers), thermos bottles or containers having elaborate check valve systems, are not readily suitable for use in restaurants, hospitals or homes.
All of the above described methods for accommodating the overpressure caused by overheating of the microwave absorbing material suffer from one or more problems, including, in some cases, the destructive, that is, irreversible, nature of the pressure relief, or the devices themselves are so complicated that both the construction and manufacturing method for making them becomes cumbersome and/or overly expensive. Alternatively, some devices rely on physical principles or robust construction, with the hope that the person heating the microwave absorbing material will not exceed expected parameters. This hope is not always borne out in reality.
While the present invention is described at least partially as a process of heating by microwaves, use of other methods of heating are also possible, for example, induction heating of foods. See, for example, U.S. Pat. No. 7,183,525 to Fuchs and U.S. Pat. No. 7,038,179 to Kim et al. While this invention relies as a best mode of heating that includes use of a microwave oven, it is conceivable that other types of indirect, quick heating may be used and or developed in the future. Thus, the source of heat provided in this invention should be understood to include any form of heating that quickly and efficiently heats up a heat absorbing material, as described below.
None of the prior art methods known heretofore teach an easy to manufacture, flexible device that can accommodate internal overpressure by opening a relief valve at the moment that the walls of the chamber begin to deform, and which exact pressure and temperature combination does not depend on preselected parameters, but depends directly on the individual characteristics of the particular device that is being heated. What is needed is a non-destructive, overpressure mechanism for use in a chamber holding thermal energy or microwave absorbing materials, that will relieve the pressure only when the structure defining the chamber begins the deformation process, and as a result of the structural characteristics and materials comprising the walls of the chamber, the chamber can return to its previous state to eliminate such deformation once the materials have cooled and the internal pressure has been reduced.